Fatty acids are carboxylic acids with long-chain hydrocarbon side groups and play a fundamental role in many biological processes. Fatty acids are rarely free in nature but, rather, occur in esterified form as the major component of lipids. Lipids/fatty acids are sources of energy (e.g., b-oxidation) and are an integral part of cell membranes which are indispensable for processing biological or biochemical information.
Fatty acids can be divided into two groups: the saturated fatty acids and the unsaturated fatty acids which contain one or more carbon double bond in cis-configuration. Unsaturated fatty acids are produced by terminal desaturases that belong to the class of nonheme-iron enzymes. Each of these enzymes are part of a electron-transport system that contains two other proteins, namely cytochrome b5 and NADH-cytochrome b5 reductase. Specifically, such enzymes catalyze the formation of double bonds between the carbon atoms of a fatty acid molecule. Human and other mammals have a limited spectrum of these desaturases that are required for the formation of particular double bonds in unsaturated fatty acids. Thus, humans have to take up some fatty acids through their diet. Such essential fatty acids, for example, are linoleic acid (C18:2); linolenic acid (C18:3), arachidonic acid (C20:4). In contrast, insects and plants are able to synthesize a much larger variety of unsaturated fatty acids and their derivatives.
Long chain polyunsaturated fatty acids (LCPUFAs) such as docosahexaenoic acid (DHA, 22:6(4,7,10,13,16,19)) are essential components of cell membranes of various tissues and organelles in mammals (nerve, retina, brain and immune cells). For example, over 30% of fatty acids in brain phospholipid are 22:6 (n-3) and 20:4 (n-6). (Crawford, M. A., et al., (1997) Am. J. Clin. Nutr. 66:1032S–1041S). In retina, DHA accounts for more than 60% of the total fatty acids in the rod outer segment, the photosensitive part of the photoreceptor cell. (Giusto, N. M., et al. (2000) Prog. Lipid Res. 39:315–391). Clinical studies have shown that DHA is essential for the growth and development of the brain in infants, and for maintenance of normal brain function in adults (Martinetz, M. (1992) J. Pediatr. 120:S129–S138). DHA also has significant effects on photoreceptor function involved in the signal transduction process, rhodopsin activation, and rod and cone development (Giusto, N. M., et al. (2000) Prog. Lipid Res. 39:315–391). In addition, some positive effects of DHA were also found on diseases such as hypertension, arthritis, atherosclerosis, depression, thrombosis and cancers (Horrocks, L. A. and Yeo, Y. K. (1999) Pharmacol. Res. 40:211–215). Therefore, the appropriate dietary supply of the fatty acid is important for humans to remain healthy. It is particularly important for infant, young children and senior citizens to adequately intake these fatty acids from the diet since they cannot be efficiently synthesized in their body and must be supplemented by food (Spector, A. A. (1999) Lipids 34:S1–S3).
DHA is a fatty acid of the n-3 series according to the location of the last double bond in the methyl end. It is synthesized via alternating steps of desaturation and elongation. Starting with 18:3 (9,12,15), biosynthesis of DHA involves Δ6 desaturation to 18:4 (6,9,12,15), followed by elongation to 20:4 (8,11,14,17) and Δ5 desaturation to 20:5 (5,8,11,14,17). Beyond this point, there are some controversies about the biosynthesis. The conventional view is that 20:5 (5,8,11,14,17) is elongated to 22:5 (7,10,13,16,19) and then converted to 22:6 (4,7,10,13,16,19) by the final Δ4 desaturation (Horrobin, D. F. (1992) Prog. Lipid Res. 31:163–194). However, Sprecher et al. recently suggested an alternative pathway for DHA biosynthesis, which is independent of Δ4 desaturase, involving two consecutive elongations, a Δ6 desaturation and a two-carbon shortening via limited β-oxidation in peroxisome (Sprecher, H., et al. (1995) J. Lipid Res. 36:2471–2477; Sprecher, H., et al. (1999) Lipids 34:S153–S156).
Production of DHA is important because of its beneficial effect on human health. Currently the major sources of DHA are oils from fish and algae. Fish oil is a major and traditional source for this fatty acid, however, it is usually oxidized by the time it is sold. In addition, the supply of the oil is highly variable and its source is in jeopardy with the shrinking fish populations while the algal source is expensive due to low yield and the high costs of extraction.
EPA and AA are both Δ5 essential fatty acids. They form a unique class of food and feed constituents for humans and animals. EPA belongs to the n-3 series with five double bonds in the acyl chain, is found in marine food, and is abundant in oily fish from North Atlantic. AA belongs to the n-6 series with four double bonds. The lack of a double bond in the ω-3 position confers on AA different properties than those found in EPA. The eicosanoids produced from AA have strong inflammatory and platelet aggregating properties, whereas those derived from EPA have anti-inflammatory and anti-platelet aggregating properties. AA can be obtained from some foods such as meat, fish, and eggs, but the concentration is low.
Gamma-linolenic acid (GLA) is another essential fatty acid found in mammals. GLA is the metabolic intermediate for very long chain n-6 fatty acids and for various active molecules. In mammals, formation of long chain polyunsaturated fatty acids is rate-limited by Δ6 desaturation. Many physiological and pathological conditions such as aging, stress, diabetes, eczema, and some infections have been shown to depress the Δ6 desaturation step. In addition, GLA is readily catabolized from the oxidation and rapid cell division associated with certain disorders, e.g., cancer or inflammation. Therefore, dietary supplementation with GLA can reduce the risks of these disorders. Clinical studies have shown that dietary supplementation with GLA is effective in treating some pathological conditions such as atopic eczema, premenstrual syndrome, diabetes, hypercholesterolemia, and inflammatory and cardiovascular disorders.
The predominant sources of GLA are oils from plants such as evening primrose (Oenothera biennis), borage (Borago officinalis L.), black currant (Ribes nigrum), and from microorganisms such as Mortierella sp., Mucor sp., and Cyanobacteria. However, these GLA sources are not ideal for dietary supplementation due to large fluctuations in availability and costs associated with extraction processes.